Methane conversion reactor

ABSTRACT

The present invention discloses a methane converter comprising a reactor tube and a heating means to raise the reactor tube temperature to the desired reaction temperature wherein the reactor tube is packed with a high temperature stable, glass fibers the fibers having a layer of catalytic material on them. The present reactor operating at temperatures below 850° C. and producing hydrogen and high density carbon. Also disclosed is a carbon dioxide reclamation system utilizing the present methane converter.

TECHNICAL FIELD

The field of art to which this invention pertains is methane reformationand carbon formation. This field also includes gas reclamation.

BACKGROUND OF THE INVENTION

In many environments it is necessary to recapture oxygen from theexhaled carbon dioxide of mammals living in the environment. The mostcommon of these situations would be a closed environment such as asubmarine or a spacecraft. Generally, the reclamation process comprisespassing the carbon dioxide gas through a concentrator and then directingthe concentrated carbon dioxide to a carbon dioxide reduction system.Typically, the carbon dioxide reduction system results in the reaction(CO₂ +4H₂ →H₂ O+CH₄). The resulting water may be electrolyzed to producehydrogen and breathing oxygen, while the methane formed may be reactedto produce carbon and hydrogen; the hydrogen being useful in theoperation of fuel cell batteries on board the vessel while the carbonwould be a dispensable by-product.

A typical method for the conversion of methane useful in theabove-mentioned system involves passing the methane either alone or incombination with another gas into a heated reactor tube and causing themethane to pyrolytically decompose to form H₂ and carbon. The carbonthen remains in the reactor until such time as the build-up reduces theflow of methane into the reactor preventing the optimum operation of thereactor and necessitating its removal.

This carbon management poses a limitation on the operation of thesystem. U.S. Pat. No. 4,452,676 attempts to solve this problem byoperating the conversion process at very high temperature, therebycreating a very dense carbon which occupies far less space in thereactor for the same quantity of methane converted, thereby allowing forlonger operating cycles between cleanings.

However, the high temperatures required to operate such a reactorcreates certain problems with the types of materials which may be usedin the manufacture of the reactor and will also require significantenergy consumption from a very limited energy source.

An alternative methane conversion system is referred to as the Boschprocess, wherein the methane conversion temperatures necessary in thereactor vessel are about 700° C., significantly lower than thetemperatures required in the prior process described above, therebyreducing the consumption of energy required for the process. The loweroperating temperatures are achieved by the introduction of an expendableiron catalyst into the reaction vessel. However, although the reactiontakes place at a lower temperature, the resulting carbon has a lowerpacking density and therefore, requires more frequent removal than theprocess which produces high density carbon.

A recent study entitled Formation of Dense Carbon on Fused-Quartz Woolfor Spacecraft Life Support Application discloses that the introductionof quartz wool into the reactor vessel will result in high densitycarbon formation and increased efficiencies for methane conversion.However, the operating temperatures were still in excess of 1000° C.

Therefore, what is needed in this art is a method for converting methanegas producing hydrogen and carbon in which the operating temperaturesare low, the carbon formed has a high density and the conversionefficiency remains high.

DISCLOSURE OF THE INVENTION

This invention is directed toward an improved methane conversion processin which the methane gas is introduced into a reactor vessel which hasbeen packed with a glass fiber material having a layer of catalyticmetal on the fibers, the reaction taking place at temperatures between750° C. and 850° C. and the resulting carbon having a high density.

Also disclosed is a methane reactor in which the reactor vessel ispacked with glass fibers having a layer of catalytic metal thereonresulting in a reactor which will convert methane gas to hydrogen andhigh density carbon at temperatures from about 750° C. to about 850° C.

Also disclosed is an improved system for the reclamation of oxygen fromcarbon dioxide exhaust gases wherein the carbon dioxide is passedthrough a concentrator and then the concentrated carbon dioxide isreacted in the presence of hydrogen to form water and methane,thereafter the water is lectrolyzed to form hydrogen and oxygen and themethane is converted to hydrogen and carbon by passing through thereactor vessel of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a carbon dioxide reclamation systemaccording to the present invention.

FIG. 2 shows a cross-sectional view of the methane reactor vessel of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The carbon dioxide conversion system for reclaiming oxygen is shown byreference to the figures. As shown therein, the process is a two stepprocess resulting in the overall production of a high density carbon andwater which may be disassociated to form hydrogen and oxygen.

The carbon dioxide is passed into the initial reactor 1 in which it ishydrogenated by contact with a hydrogenation catalyst (such as HamiltonStandard UASC 151G - 20% by weight ruthenium on alumina granules) toform methane and water as described in the reaction: CO₂ +4H₂ →CH₄ +2H₂O. These reaction products are then cooled to condense out the water andis separated using a conventional condenser/separator 2 (note for spaceapplications a porous plate or rotary inertial condenser/separator ispreferred).

The remaining reactant gas (methane) is then directed to a second stagereactor 3 in which the methane is converted to hydrogen gas and highdensity carbon. It may be desirable to pass the gas through a heatingmeans such as a heat exchanger 4 prior to passing it into the secondstage reactor). This reactor comprises a reactor vessel of thermallystable, material (typically a high temperature glass) , packed withthermally stable, high temperature glass fibers having a layer ofcatalytic metal on them.

FIG. 2 shows a typical configuration of the proposed methane reactors.The reactor components are conventional and comprise a reactor vessel 6in which the methane is introduced through an opening 8, and thehydrogen is removed through exit port 14. The reactor vessel isgenerally mounted in an electrically or heat insulating means 13. Thereactor vessel is typically surrounded by a heating means 10 by whichthe temperature of the reactor vessel may be raised and maintained atthe desired reaction temperature and some insulating materials 11 toensure the efficient operation of the heater. Methane converters such asthese are described in U.S. Pat. No. 4,452,676. The present inventionintroduces a packing of glass fibers 12 having a layer of catalyticmetal on them which reduces the temperature at which the methanereaction takes place yet continues to form high density carbon at highconversion rates.

As has been stated, the basic components of the methane reactor, theheater, insulation, and reactor vessel are conventional. The heater canbe by any conventional heating means such as a resistance furnace, whichis preferred. An exemplary resistance furnace setup includes the use offirebrick and Globar silicon carbide rods available form Norton Company,Worcester, Mass. If a resistance heater is used, the coils are wrappedaround the reactor vessel and then sufficient current supplied to theresistance heater from any conventional power source to produce thedesired temperature.

The reactant vessels may be any shape which will lend itself to beingpacked with the catalyzed glass fibers, however, the preferred shape isin the form of a tube. The vessels should be formed of high temperature,thermally stable and chemically inert materials which do not lose theirshape or deform at the operating temperatures of about 750° C. to about850° C. These may be high temperature glasses, ceramics or metals. Thepreferred materials are high temperature quartz and high silica glasses(such as Vycor glass) available from Corning Glass Works, Corning, N.Y.The internal diameter of the vessel may be any size but typically itshould range from about 20 centimeters to about 25 centimeters. Thereason glass is preferred is that the carbon formed during the processis easily removed from the glass; whereas if metal components were usedthe carbon would adhere too strongly to the metal part and the entirereactor tube would have to be replaced when it was time to remove thecarbon.

The reactant vessel is then packed with glass or glass-like fibershaving a layer of metallic catalyst on the glass fibers (for purposes ofthis discussion glass and glass-like fibers are used interchangeably).The glass fibers should be thermally stable at the conversiontemperatures, inert to the reaction and should have a high surface tovolume ratio. Typical glass fibers useful in the operation of thisinvention are fused-quartz glass fibers, borosilica fibers etc., withthe preferred being the fused-quartz fibers, such glass fibers shouldhave a melt temperature and be stable at temperatures in excess of 850°.These fibers preferably will have surface to volume ratios of at least1000 square centimeters surface area per cubic centimeter of volume withabout 4000 to about 8000 square centimeters surface area per cubiccentimeter packed volume being preferred. Additionally, although thediameter of the fibers is not limited, it is preferred that the averagediameter of the glass fibers be about 2 micrometers to about 10micrometers. Both the surface to volume ratio and the fiber diameterpreferences are meant to enhance the reactive surface inside the reactorvessel. These glass fibers are generally in the form of a loosely wovenmat; however, other configurations may be used so long as the fibers arecoated with a metal catalytic layer and are capable of being compactedand placed into the reactor vessel without much destruction of thefibers.

These glass fibers must have a metal catalytic layer on them. This layermay be placed on the fibers using any conventional technique such aschemical vapor deposition from carbonyls of the metals, or electrolessplating. The preferred method is through chemical vapor deposition ofthe metal carbonyls run in a mass transfer rate--limited regime toensure uniformity of coating throughout the glass fiber thickness. Theseprocesses are well known to those skilled in the art and need not bedetailed here. The metal catalytic layer formed should be selected fromthe group iron, cobalt, nickel, or palladium with nickel beingpreferred. The thickness or amount of catalytic material placed on theglass fiber substrate is not critical, however, the more catalyticsurface area available the more efficient the conversion is likely tobe. Typically the amount of catalyzed metal should be about 33% to about67% weight fraction of the glass.

The amount and density of the packing material is limited by the abilityof the methane to easily pass through the reactor vessel. The packingdensity of the glass fibers should be about 1% to about 4% by volume ofthe reactor vessel volume and about 1% to about 2% by volume of thereactor vessel being most preferred.

The operation of this methane converter may be followed in FIG. 2. Themethane gas is introduced into the reactor vessel 6 through inletopening 8 and flows under pressure through the reactor vessel 6. As itpasses through the reactor vessel 6, it contacts the catalytic surfaceof the glass fibers 12 which have been heated to a temperature ofbetween 750° C. and 850° C. The catalytic layer causes the methane tospontaneously breakdown to form high density carbon and hydrogen gas.The carbon is deposited onto the glass fibers and the hydrogen passesout of the reactor vessel through outlet 14.

The flow rate and the pressure at which the methane is forced throughthe reactor vessel will be a function of the packing density of thereactor vessel and the conversion efficiency of the reactor. Theseparameters will be easily discernible from simple experimentation.

EXAMPLE

A reactor was constructed having a reactor vessel of quartz glass in theform of a tube 60 centimeters long having an internal diameter of 4.5centimeters and a wall thickness of 0.15 centimeters. The tube waspurchased commercially from Quartz Scientific, Inc., Fairport Harbor,OH. The tube was heated by a molybdenum disilicide electrical resistanceheater elements in a 10 centimeter high by 15 centimeter square heatedzone of a fibrous-alumina insulated box furnace, built for methaneconversion reactor testing. The reactor vessel was packed to a volumedensity of 2% with a 30 centimeter wide fused quartz wool mat rolled upand inserted into the center of the vessel. The glass mat was 1.0centimeter thick and had an average fiber diameter of 9 micrometers. Thequartz wool was purchased commercially from Quartz Products, Inc.,Plainfield, N.J. The glass mat had a layer of nickel metal deposited onthe glass fiber surface by chemical vapor deposition from nickelcarbonyl. The average thickness of the metal layer was 1.0 micrometers.

The reactor was operated at a temperature of 850° C. and a methane flowrate of 100 sccm (standard cubic centimeters per minute). The conversionrate for the methane and the density of the carbon formed were comparedto the prior art methods under their optimum conditions and the resultsare shown in the Table below.

                  TABLE                                                           ______________________________________                                        DENSITY   °C.               BULK                                       REACTOR   TEMPER-   % CONVERSION   DENSITY                                    PACKING   ATURE     SINGLE PASS    LB/FT3                                     ______________________________________                                        uncatalyzed                                                                             1200      75             45                                         quartz wool                                                                   catalyzed  850      80             36                                         quartz wool                                                                   Bosch Method                                                                             650       6             20                                         unpacked  1200      56             18                                         quartz tube                                                                   ______________________________________                                    

As may be seen form the above example, the present inventionsignificantly lowers the reaction temperature at which the conversiontakes place while maintaining a high conversion efficiency of themethane and high packing density of the resulting carbon. This resultsin a more efficient system which uses less energy yet still produces anefficient, long term, life cycle for the reactor. Additionally, the lowtemperatures at which these reactors operate allows for the use of lessexotic, expensive and heavy materials in the construction of the reactoritself and also will prolong the useful life of the reactor vessel by asmuch as 100 times that of life expectancy when operated at the highertemperatures.

It should be understood that the invention is not limited to theparticular embodiments shown and described herein, but that variouschanges and modifications may be made without departing from the spiritand scope of this novel concept as defined by the following claims.

I claim:
 1. A method of reclaiming oxygen from carbon dioxide exhaustcomprising reacting the carbon dioxide with hydrogen to form a mixtureof water and methane and electrolyzing the water to form hydrogen gasand oxygen and then introducing the methane into a methane converterhaving a heated reactor vessel where the methane is converted to carbonand hydrogen wherein the improvement comprises contacting the methane insaid reactor vessel with a high temperature stable glass fiber materialhaving a layer of a metallic catalyst thereon, at a temperaturesufficient to cause the methane to catalytically decompose forminghydrogen gas and carbon.
 2. The method of claim 1 wherein the reactionvessel is heated to a temperature of about 750° C. to about 850° C. 3.The method of claim 1 wherein the high temperature glass fibers have anaverage diameter of from about 2 micrometers to about 10 micrometers. 4.The method of claim 1 wherein the high temperature glass fibers arefused-quartz wool.
 5. The method of claim 1 wherein the catalyst appliedto the high temperature glass fibers is selected from the groupconsisting of cobalt, nickel, iron, and palladium.
 6. The method ofclaim 5 wherein the catalyst metal is present as a weight fraction ofthe high temperature glass fibers from about 33 percent to about 67percent.